Last night I had a dream where I was Stan in a GTA styled environment and my friend was Cartman in a tank. It was pretty much like GTA 4 free roam except so much more bad*** because we were South Park characters and the dialogue between us was just like South Park. My friend(Cartman was kicking my *** and then killed Kyle(played by a random person) screaming: Haha stupid J*w!) and I being Stan hopped in a helicopter and shot Cartman down, that was the end of my dream. I thought it was pretty cool and might be an awesome idea for a future video game(seeing that South Park is always trying to come out with games.)... It probably sounds stupid but if I think it's an awesome idea and if I had the capabilities to do it i'd be making that game right now as we speak.

An old fan-made PC game called Kenny's Rampage II had a mini-game within it where you played as Stan in the original GTA game from 1997. That's the closest thing you'll get unless you install GTA IV on your computer and modify the main character in a script-writing program to look like Stan.